Studies completed in this project indicate that cytoplasmic membranes prepared from sensitive bacteria treated with colicins K or E1, but not E2 or E3, are defective in their ability to catalyze ATP-linked transhydrogenase. Neither the electron transport system nor ATPase hydrolytic activity nor the treated membranes were affected. Thus the membrane defect appears to be due to an impairment in energy coupling. The colicin effect on the membrane is correlated with colicin lethality; it is not expressed in tolerant or anaerobic sensitive bacteria which bind colicin but are not killed by it. Because the altered membrane function may reflect an altered membrane structure, the purified membranes from treated and untreated bacteria will be analyzed for lipid and protein composition using thin layer chromatography and polyacrylamide gel electrophoresis. Reconstitution experiments and untreated bacteria are depleted of the energy transducing ATPase enzyme complex. The activity will be reconstituted in the depleted membranes by the addition of the extracted ATPase enzyme of the treated and untreated membranes. The results of these experiments should narrow the search for the membrane defective components. Characterization of in vitro effects of colicins on energy dependent transhydrogenase are in progress.